1) Field of the Invention
This invention relates to a high pressure fuel injection pipe for a fuel supply passage of a diesel engine, and more particularly to a small thick-walled metal fuel injection tubing having an outside diameter of at most about 30 mm and having its outer and inner tubular members metallurgically press-fitted on each other, which is excellently resistant to cavitation erosion and pressure. The invention relates also to a process of producing the small thick-walled metal fuel injection tubing.
2) Description of the Related Art
Generally, in a high pressure fuel injection pipe of the described type such as for a diesel engine, high pressure fuel flows inside the injection pipe under the conditions, i.e. an injection time of about 5 milliseconds, a flow speed of at most about 15 m/sec and an internal pressure of 200-600 kg/cm.sup.2 (peak pressure); either flow speed or internal pressure fluctuates frequently and sharply.
As can be seen in, for example, a tubing for a high pressure fuel pipe, this type of composite metal tubing has heretofore been formed by a procedure which comprises the press-fitting of large and small diameter high pressure piping carbon steel pipes (JIS G 3455 STS 38), each having a copper plating film preliminarily provided around the circumferential mating surface thereof, and subsequent heat-treating of them to effect mutual brazing with the copper plating films as the filler which are present around the circumferential mating surfaces, or by a simple procedure which comprises inserting one of two pipes having mutually different diameters into the other and mutually press-fitting them by solid drawing or the like.
However, according to the former of these conventional procedures, complicated steps are needed because copper plating must be applied around the inner surface and the outer surface of the pipes. If the brazing is inadequate, the mutually press-fitted mating circumferential surfaces undergoes a relaxture phenomenon to create a gap from which portion a possible breakage due to fatigue tends to progress. Yet if the brazing is adequate, only a limited degree of resistance against vibration can be obtained as the base metal becomes soft due to the heat during brazing. Therefore an excellent composite fuel injection tubing cannot be achieved.
According to the latter conventional procedure, because the resulting configuration has the mating faces simply subjected to press-fitting, the mating circumferential interface disadvantageously undergoes a relaxture phenomenon leading to reduction of the mechanical strength when the composite tubing is used as a fuel injection pipe. Coupled with vibration during rotation of an engine, this often gives rise to cracking or breakage.
As countermeasures have been made in recent years in increasing the power as well as in reducing NOx and minimizing black smoke, the tendency of highly pressurizing fuel is particularly on the rise. A high pressure fuel injection tubing has thus been demanded which can be used under the conditions, e.g. an injection time of 1-2 milliseconds, a flow speed of at most about 50 m/sec and an internal pressure of 600-1,000 kg/cm.sup.2 (peak pressure). Consequently, under such severe conditions, a high pressure fuel injection pipe most highly requires (1) durability against fatigue due to the repeated high pressure loading, (2) resistance to cavitation erosion, and (3) resistance against vibration from the vehicle body. So safety measures against cracking and breakage of the injection pipe has been very significant.
To this end, as an attempted measure against the severe conditions, it has been proposed that a small diameter thick-walled pipe having a wall thickness of 25-40% of its outside diameter is used for the inner tubular member of a high pressure fuel injection pipe, that the inner wall surface of the inner tubular member has no irregular wavy swells which would be a cause for development of fuel eddy resistance, and that there is eliminated a gap, between the thick-walled inner tubular member and the outer tubular member, which would cause a breakage of the inner tubular member and/or fuel leak. However, this increased eddy resistance creates cavitation erosion in the inner wall surface of the inner tubular member, thus resulting in a breakage of the inner tubular member.
Therefore, a fuel injection pipe for diesel engine usable in safety under the above-mentioned severe conditions has not yet been developed for the present.